1. Field of the Invention
This invention relates to extrusion die apparatus for the extrusion of thermoplastic material in a tubular form and in particular to such apparatus for the extrusion of multilayered tubing. In the description below and in the drawings the apparatus of the present invention is described and shown as it is used for the manufacturing of film by the blown film process, but the apparatus is not so limited but can be used also for the manufacturing of thermoplastic piping and tubing.
2. Description of the Prior Art
The manufacturing of thermoplastic film by the blown film process is well known in the art and reference may be had to U.S. Pat. No. 2,461,975 (Fuller) for a disclosure of the basic process. As shown in this patent, molten plastic material is introduced into an annular die and is extruded therefrom in the form of a tube. The die comprised basically an outer annular body within which is mounted an inner cylindrical mandrel. The plastic flows through the annular space between the body and the mandrel and is changed in form from a cylindrical rod to a tube of the desired diameter.
In order to obtain satisfactory gauge control of the extruded tube, to obtain temperature homogeneity of the material and to eliminate weld lines in the material it has been found desirable to modify the configuration of the outer surface of the mandrel. One of the most successful of these configurations is the incorporation of one or more helical or spiral grooves on the mandrel with the depth of the groove or grooves gradually decreasing toward the downstream end of the die with the grooves ending at some distance from the die orifice. Mandrels with this configuration are generally known as "spiral mandrels" and so shall be called here. With the spiral mandrel, the plastic material not only travels directly downstream in the annulus between the body and mandrel, but a major portion moves in the helical grooves from which, as the groove depth decreases, the material is forced out of the groove and again travels in a generally downstream direction. By this means the material is continuously mixed and blended resulting in a homogeneous condition and elimination of weld lines. This configuration, the spiral mandrel, is well known in the art and is disclosed in U.S. Pat. Nos. 2,260,750 (Kopitke); 2,794,213 (Davis); 3,267,194 (Stenger); 3,376,605 (Beattie); 3,689,192 (Upmeier); and British Pat. No. 823,066.
It is also known in the art to extrude tubing comprising two or more layers of plastic materials, the process being known as coextrusion. The same basic type of extrusion die as above is used with an outer annular body and two or more internal mandrels forming concentric annular passages between the body and mandrel and between the inner mandrels. Reference may be made to Papesh U.S. Pat. Nos. 3,649,143 and 3,966,861 for a disclosure of the apparatus involved in which the use of spiral mandrels is also disclosed.